The objective of this research project is to develop self-consistent field (SCF) methods for the study of large biologically-related molecules and to apply these methods to problems of biological interest. For example, these problems, which will be discussed in more detail later, will include the calculation of the ground-state electronic distribution, localized molecular orbitals and various one-electron properties of biologically-related molecules. We hope to eventually extend our previously developed SCF methods to the study of the electronic distribution in the vicinity of the active sites of selected enzymes, with the hope of correlating electronic density around the active site with the function of the enzyme. Our plan of attack is to extend the partial retention of diatomic differential overlap - self - consistent field (SCF) method to handle the second-row atoms phosphorous, sulfur, and chlorine. These atoms are most important in biochemical systems.